In the past, hydrocarbon conversion reactions such as alkylations, acylations, rearrangements, and isomerizations were carried out in the presence of strong Lewis acids or strong Bronsted acids. A Lewis acid is a compound in which the normal electronic grouping about the central atom can accept an electron pair from a Lewis base which is any compound capable of donating an electron pair. Such Lewis acids include fluorides, chlorides, and bromides of aluminum, zinc, titanium, zirconium, antimony and iron. Bronsted acidity reflects the ability of a protonic acid to transfer a proton to a base. Bronsted acids suitable as catalysts for hydrocarbon conversion reactions include strong acids such as liquid sulfuric acid, hydrogen fluoride, phosphoric acid and trifluoromethanesulfonic acid. Generally these protonic acids have an H.sub.o value on the Hammett scale of -11 or less. The effectiveness of such Lewis acid and Bronsted acid catalysts for hydrocarbon conversions is directly related to the acidity of the catalyst material toward the hydrocarbon substrate.
Although these Lewis acids and Bronsted acids perform satisfactorily as catalysts for hydrocarbon conversion reactions they suffer from certain disadvantages. Lewis acids form colored hydrocarbon complexes with the anionic components formed during the hydrocarbon conversion reaction and molar amounts are normally required. Work-up is also needed to decompose these complexes and the catalyst is usually nonrecoverable. Strong protonic Bronsted acids, on the other hand, form insoluble sludges as products during the reaction which are extremely difficult to separate.
In order to overcome these disadvantages, solid acids such as perfluorinated polymersulfonic acids have been employed as catalysts for hydrocarbon conversion reactions. Such acids have acidities of greater than -11H.sub.o on the Hammett scale. These solid "super" acids do not form complexes or insoluble sludges and can be easily separated from the reaction mixture by filtration and regenerated without loss of activity. In addition these super acids can be used in catalytic amounts, are highly stable and maintain long range catalytic activity.
Solid superacidic perfluorinated polymersulfonic acids, sold under the trademarks NAFION or PFIEP by E. I. Dupont De Nemours, have been applied in organic synthesis for a variety of reactions including the alkylation of arenes with alkenes, alkyl halides, alcohols and esters; transalkylation of arenes with polyalkyl benzenes; acylation, sulfonation, and halogenation reactions; preparation of acetals, the synthesis of cyclic ethers; the preparation of .alpha.,.beta.-unsaturated carbonyl compounds and methoxy methyl ethers; the preparation of 1,1-diacetates from aldehydes; the pinacolone rearrangement; the hydration of lower olefins, the nitration of aromatic compounds; and the isomerization, disproportionation and transmethylation of methylbenzene. Common to all these acid catalyzed reactions is the formation of carbocations as intermediates.
Perfluorinated polymersulfonic acids of the NAFION or PFIEP type contain a repeating structure represented by either of the following formulae: ##STR1## wherein the ratio of x over y varies from 2 to 50 and m is 1 or 2. The above formulae (I) and (II) are in accordance with the specification of Great Britain Pat. No. 2,082,178. The molecular weight of these polymers range from about 1000 to 500,000 daltons and the sulfonic acid groups comprise from about 0.01 to 5 mequiv/gram of catalyst.
The catalysts have been prepared by polymerizing the corresponding perfluorinated vinyl compounds according to the methods disclosed in U.S. Pat. Nos. 3,282,875 and 3,882,093. U.S. Pat. No. 4,041,090 discusses the structure and describes a method for preparing the perfluorinated polymersulfonic acid by copolymerizing the corresponding perfluorinated vinyl ethers with perfluoroethylene and/or perfluoro alpha olefins.
It has now been shown in Ser. No. 705,988 now U.S. Pat. No. 4,585,750, that the acidity of such a perfluorinated polymersulfonic acid can be significantly enhanced by treating the solid surface of a carefully dried resin with certain anion-stabilizing solvents to form a new catalyst or phase.
According to the present invention, that superacidic catalyst or phase can be prepared directly from the hydrated perfluorinated polymersulfonic acid, by treatment with a mixture of polyfluorinated carboxylic acid and the corresponding anhydride.